Aaron Smith the best in the world - Gatland

British and Irish Lions coach Warren Gatland has hailed All Blacks scrum-half Aaron Smith as the leading player in world rugby.

Gatland was reappointed as coach of the Lions earlier this week, ahead of next year's tour of New Zealand, his home country.

The 52-year-old had the opportunity to take a close look at the world champions on Saturday, as he took in their Rugby Championship fixture against Argentina in Hamilton.

Speaking to Fox Sports ahead of the game, Gatland was asked for his opinion on an All Blacks side that has shone in recent months despite the international retirements of a host of key figures, including Richie McCaw, Dan Carter and Ma'a Nonu.

"I think they've taken the game on to another level," said Gatland.

"There's one player who in my opinion is the driving force behind that and is key, and that's Aaron Smith.